Multistage Compressor

ABSTRACT

A multistage compressor including: at least a first compressor main body and a second compressor main body configured to suck compressed gas discharged from the first compressor main body to discharge the compressed gas with higher pressure; a first driving source configured to drive the first compressor main body; a second driving source configured to drive the second compressor main body; an intermediate pipe configured to connect a discharge side of the first compressor main body and a suction side of the second compressor main body; a low-pressure side discharge piping system branched from the intermediate pipe; an on-off valve disposed in the low-pressure side discharge piping system and configured to switch permission and prohibition of a flow of compressed gas discharged from the first compressor main body; and a control unit configured to control driving of the first driving source and the second driving source.

INCORPORATION BY REFERENCE

The present application claims priority from Japanese application JP-2016-051854 filed on Mar. 16, 2016, the content of which is hereby incorporated by reference into this application.

TECHNICAL FIELD

The present invention relates to a multistage compressor and relates to a multistage compressor including a plurality of compressor main bodies.

BACKGROUND OF THE INVENTION

A known gas compressor configured to suck gas (for example, air) to generate compressed gas (for example, compressed air) includes a multistage compressor that connects gas passages of a plurality of compressor bodies in series (hereinafter referred to as “multistage compressor”). In some multistage compressors, a gas discharge side of a low-pressure stage compressor main body and a gas suction side of a high-pressure stage compressor main body are connected through an inter cooler (intercooler), and gas is compressed in two-stages.

In a so-called single stage compressor having one stage compressor, there is no intercooler, and accordingly there are few piping parts, so it can be said that this configuration is advantageous mainly in terms of cost.

Meanwhile, it can be said that a two-stage compressor has a compression process close to isothermal compression as compared with the single stage compressor, so the compression efficiency can be increased and the temperature of each component such as a rotor and a casing becomes lower, and thus there is also little constraint on a heat resistance about materials and the like.

For the two-stage compressor, it is known that when the compression ratio of a first stage side compressor (low-pressure stage compressor) and the compression ratio of a second stage side compressor (high-pressure stage compressor) are made equal, the highest efficiency is achieved. In the general design of a two-stage compressor, the speed of each stage is thus decided such that the compression ratio of a low-pressure stage compressor and the compression ratio of a high-pressure stage compressor become equal at a rated point.

Meanwhile, a lot of adjustable-speed apparatus are used in recent years which control a gas flow by changing the operating speed of a compressor when necessary quantity of gas varies from a viewpoint of energy saving. In this case, since an intermediate pressure loss changes with a change of quantity of gas, if the quantity of gas decreases, for example and an intermediate pressure loss falls, the suction pressure of a high-pressure stage compressor will become high. Operations are performed under pressure conditions different from the rated point. As a result, the optimal speed ratio of the low-pressure stage compressor and the high-pressure stage compressor differs from the rated point. Generally, in a two-stage compressor, the number of driving sources (for example, motor) is one, and since transmitting power to a low-pressure stage compressor and a high-pressure stage compressor is performed by the gear having a constant speed increasing ratio with respect to each stage compressor, the speed ratio of a low-pressure stage compressor and a high-pressure stage compressor cannot be changed. Therefore, there is a case that the operation at the optimum speed ratio is not performed at the time of using the partial load.

As a measure, which optimizes the operation speed ratio of the low-pressure stage compressor and the high-pressure stage compressor, there is a configuration in which the compressor main body of each stage is independently driven. JP 3352187 B2 discloses an operation method in which each of a low-pressure stage compressor main body and a high-pressure stage compressor main body is provided with a motor, which allows an independent drive, the intermediate pressure is detected, and the rotational speed is adjusted based on the detection pressure. For example, when the amount of air used decreases and the rotation speed decreases, the intermediate pressure becomes high. Therefore, when the pressure exceeds a certain set pressure, a control signal is outputted to an inverter from a controller so as to decrease the rotational speed of a low-pressure stage compressor main body or increase the rotational speed of a high-pressure stage compressor main body. According to JP 3352187 B2, the compression ratio of each stage can be made equal at the time of partial load also, and efficient operation will be achieved.

SUMMARY OF THE INVENTION

JP 3352187 B2 requires two motors, which are main parts of the compressor system. In addition, when increasing the speed by gears or belts, the number of these parts also increases. Thus, independent control of the rotational speed of each stage only for optimization of intermediate pressure increases the number of parts, and various problems such as the increase in material cost, required space, and also maintenance cost remain.

Meanwhile, some compressor users selectively use high-pressure gas and low-pressure gas depending on the application. In this case, compressors for high pressure and low pressure may be provided separately, but there are problems of increase in installation area or mounting area and increase in cost.

In this regard, one compressor can allow the selective use of both compressors. For example, high-pressure compressed gas is generated from a compressor, and this gas is stored in a reservoir tank, and when a low-pressure compressed gas is required, the pressure is reduced to a desired pressure before use. Reduced pressure in this manner results in loss of the high-pressure compressed gas energy.

A technique of obtaining compressed gas having different pressures more conveniently and efficiently is desired.

In order to solve the above problem, for example, the configuration described in the claims is applied. That is, the configuration is a multistage compressor including: at least a first compressor main body 1 and a second compressor main body 2 configured to suck compressed gas discharged from the first compressor main body 1 to discharge the compressed gas with higher pressure; a first driving source 3 configured to drive the first compressor main body 1; a second driving source 4 configured to drive the second compressor main body 2; an intermediate pipe configured to connect a discharge side of the first compressor main body 1 and a suction side of the second compressor main body 2; a low-pressure side discharge piping system branched from the intermediate pipe; an on-off valve 13 disposed in the low-pressure side discharge piping system and configured to switch permission and prohibition of a flow of compressed gas discharged from the first compressor main body 1; and a control unit 10 configured to control driving of the first driving source 3 and the second driving source 4, wherein the control unit 10 is configured to drive the first driving source 3 alone when the on-off valve 13 is opened.

Additionally, for example, another configuration is a multistage compressor including: at least a first compressor main body 1 and a second compressor main body 2 configured to suck compressed gas discharged from the first compressor main body 1 to discharge the compressed gas with higher pressure; a first driving source 3 configured to drive the first compressor main body 1; a second driving source 4 configured to drive the second compressor main body 2; an intermediate pipe which connects a discharge side of the first compressor main body 1, and a suction side of the second compressor main body 2; a low-pressure side discharge piping system branched from the intermediate pipe; an on-off valve 13 disposed in the low-pressure side discharge piping system and configured to switch permission and prohibition of a flow of compressed gas discharged from the first compressor main body 1; a by-pass pipe 25 configured to a discharge pipe of the second compressor main body 2 and a downstream side of the on-off valve 13 of the low-pressure side discharge pipe; a by-pass valve 26 configured to switch permission and prohibition of a flow of compressed gas into the by-pass pipe 25, the compressed gas flowing through the discharge pipe; a suction pipe connected to the intermediate pipe or a suction side of the second compression main body and in which gas flows without passing through the first compressor main body 1; a valve body 36 disposed at the suction pipe, and configured to permit or prohibit a flow of the gas; and a control unit 10 configured to control driving of the first driving source 3 and the second driving source 4, wherein the control unit 10 configured to drive the second driving source 4 alone when the on-off valve 13, the by-pass valve 26, and the valve body 36 are opened.

According to the present invention, high-pressure and low-pressure compressed gases can be efficiently generated. Other problems, configurations, and effects of the present invention will become clear from the description below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram schematically showing a configuration of an oilless screw compressor according to Embodiment 1 to which the present invention is applied;

FIG. 2 is a cross-sectional view showing a compressor body configuration of an oilless screw compressor in detail, and a schematic view of its control system diagram according to Embodiment 1;

FIG. 3 is a block diagram schematically showing a configuration of an oilless screw compressor according to Embodiment 2 to which the present invention is applied; and

FIG. 4 is a block diagram schematically showing a configuration of an oilless screw compressor according to Embodiment 3 to which the present invention is applied.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of the present invention will be described with reference to the drawings.

Embodiment 1

FIG. 1 is a schematic view of a two-stage screw air compressor (hereinafter simply referred to as “compressor”) 20A, and a configuration of a compressor system (including a high-pressure/low-pressure air lines, a reservoir tank 8 and the like), and a gas flow according to Embodiment 1 to which the present invention is applied. Although an oilless screw air compressor is applied in this embodiment, the present invention is not limited thereto, and a liquid supply (oil, water, etc.) type compressor may be applied. Compression mechanism such as scroll, reciprocation, vane or the like may be applied, and multistage configurations from a combination of various types and the like may be applied. Various configurations are applicable within a scope not deviating from the spirit thereof.

The compressor 20A includes a low-pressure stage compressor main body 1, a driving source (for example, motor 3) configured to drive the low-pressure stage compressor main body 1, a high-pressure stage compressor main body 2, a driving source (motor 4) that is configured to drive the high-pressure stage compressor main body 2, an intercooler 6, and aftercooler 7. A piping system of the discharge side of the low-pressure stage compressor main body 1 is connected with a piping system of the suction side of the high-pressure stage compressor main body 2 in series. The low-pressure stage compressor main body 1 compresses the suction atmosphere from the outside, and the discharged compressed air is intercooled by the intercooler 6. Then, the high-pressure stage compressor main body 2 sucks this cooled air, and further pressurizes it, discharges more high-pressure compressed air, and the resultant compressed air is cooled by the aftercooler 7 to a predetermined temperature.

The intermediate piping system connected to the high-pressure stage compressor main body 2 from the intercooler 6 branches from its midway. One branch pipe is provided with a low-pressure stage discharge check valve 19 and a low-pressure line discharge valve (on-off valve) 13, and is connected to one or more low-pressure air lines 100 to supply low-pressure compressed air to customers. The low-pressure line discharge valve 13 and the low-pressure stage discharge check valve 19 permit or prohibit flow in the low-pressure air line 100 from an intermediate pipe.

The other branch pipe is connected to the suction side of the high-pressure stage compressor main body 2 through a high-pressure stage suction valve 14. The discharge side piping system of the high-pressure stage compressor main body 2 is provided with a high-pressure stage discharge check valve 18 and the aftercooler 7 where the high-pressure stage discharge check valve 18 is disposed upstream of the aftercooler 7, and the discharge side piping system is connected with the reservoir tank 8 disposed outside of the compressor 20A. The reservoir tank 8 is connected with one or more high-pressure air lines 200 to supply high-pressure compressed air to customers.

The compressor 20A includes a pressure sensor 21 that detects the discharge pressure of the low-pressure stage compressor main body 1 and which is disposed between the downstream side of the intercooler 6 and the branch point of the intermediate pipe. The reservoir tank 8 is provided with a pressure sensor 22 which detects the internal pressure of the reservoir tank 8. The pressure sensor 22 may be disposed in a piping system downstream from the high-pressure stage discharge check valve 18 in the compressor 20A. The detection pressure of each pressure sensor 21 and 22 is inputted into a control unit 10 which is communicably connected with each pressure sensor 21 and 22, and the control unit 10 controls the drive rotation of the motors 3 and 4 according to these pressure values. In this embodiment, inverters 11A and 11B are provided for the motors 3 and 4, respectively, and the control unit 10 performs variable speed control through the inverters.

FIG. 2 shows a specific configuration of each of compressor main bodies, motors and the like on the low-pressure stage side and the high-pressure stage side. The low-pressure stage compressor main body 1 includes a pair of male and female screw rotors 51 a and 51 b in a compression chamber formed in a compressor main body casing 50. The respective screw rotors mesh with each other in a non-contact manner through a predetermined gap, and the rotors and the inner wall of the compression chamber serve as a compression operating chamber. The shaft of the male rotor is formed in series with a main shaft 52 of the motor 3 in the coaxial direction, and a timing gear 53 which meshes with the gear installed at the end portion of the female rotor to transmit power is disposed at the opposite side end portion of the motor 3 in the axial direction. The compressor main body casing 50 and a motor casing 55 are integrally connected along the shaft. Although the rotor 51 a and a main shaft 52 of the motor are directly connected in this embodiment, gear connection or belt drive may be applied. Further, although the motor 3 and the like are a radial motor as an example, other types of motors such as an axial motor and the like may be used.

The high-pressure stage compressor main body 2 has substantially similar configuration as the low-pressure stage compressor main body 1 except that the high-pressure stage compressor main body 1 has a smaller compression volume and a higher number of revolutions.

As mentioned above, the motors 3 and 4 disposed in each compressor main body receive power supply from the inverters 11 a and 11 b, respectively. Each of the inverters 11 a and 11 b is configured so that the control unit 10 can independently transmit rotational frequency command values. Accordingly, the low-pressure stage compressor main body 1 and the high-pressure stage compressor main body 2 can be independently controlled.

In this regard, for example, in the case of a multistage compressor having one drive source and configured to simultaneously drive the low-pressure side and the high-pressure side via a gear or belt, the rotational drive ratio of each compressor body is fixed, and it is impossible to perform drive control of either one alone. However, the compressor 20A of the present embodiment is characterized in that the drive control of either one alone is possible.

The control unit 10 is a function part in which cooperation of an arithmetic unit and a program is performed. It has an external input/output interface (not shown), receives various inputs relating to setting values, switching of operation modes and the like, and can appropriately outputs driving conditions and various control information.

The control unit 10 is communicatively connected with the pressure sensors 21 and 22 (dot-and-dash line in the figure), and performs control according to the detection pressure. Further, it is connected to a solenoid valve such as a low-pressure line discharge valve 13 and a high-pressure stage suction valve 14 (dotted line in the figure) to control opening and closing operations of these valves.

One feature of the compressor 20A having such a configuration is that it can use separately both of low-pressure compressed air generated by the low-pressure stage compressor main body 1 alone, and high-pressure compressed air generated from the low-pressure stage compressor main body 1 and the high-pressure stage compressor main body 2 to supply each air to customers. Specifically, (1) when using low-pressure compressed air generated by the low-pressure stage compressor main body 1 alone, the low-pressure line discharge valve 13 is “opened”, the high-pressure stage suction valve 14 is “closed”, and the operation of the high-pressure stage compressor main body 2 is stopped. (2) When using high-pressure compressed air, the low-pressure line discharge valve 13 is “closed” and the high-pressure stage suction valve 14 is “opened”, and both of the low-pressure stage compressor main body and the high-pressure stage compressor main body are operated. (3) When using the compressed air of both of the low-pressure stage compressor main body 1 and the high-pressure stage compressor main body 2, both of the low-pressure line discharge valve 13 and the high-pressure stage suction valve 14 are “opened”, and both of low-pressure stage compressor main bodies 1 and high-pressure stage compressor main bodies 2 are operated.

Hereinafter, each operation mode is specifically described.

When used as an ordinary two-stage compressor, operation is performed such that the low-pressure line discharge valve 13 is “closed” and the high-pressure stage suction valve 14 is “opened”. All the compressed air discharged from the low-pressure stage compressor main body 1 is sucked by the high-pressure stage compressor main body 2, is further compressed into high pressure, and is discharged by the reservoir tank 8. In this case, the operating speed of the low-pressure stage compressor main body 1 and the high-pressure stage compressor main body 2 is changed according to used air flow rate. For example, when the amount of used air is small, since the intermediate pressure becomes higher than that of the rated operation, if the ratio (n1/n2) of speed n1 of the low-pressure stage compressor main body 1 to speed n2 of the high-pressure stage compressor main body is made small relative to the rating, intermediate pressure can be lowered appropriately.

Next, when using the compressed air alone which the low-pressure stage compressor main body 1 generates, the low-pressure line discharge valve 13 is “opened”, the high-pressure stage suction valve 14 is “closed”, and operation of the high-pressure stage compressor main body 2 stops. Accordingly, high-pressure stage compressor main body 2 is separated from the operation system, and the compressed air can be supplied only to the low-pressure air line 100.

Finally, when compressed air of the low-pressure stage compressor main body 1 and compressed air generated by the high-pressure stage compressor main body 2 are used at the same time, both the low-pressure line discharge valve 13 and the high-pressure stage suction valve 14 are opened”.

Here, when supplied to the low-pressure air line from the low-pressure stage compressor main body 1, intermediate pressure will lower according to the supplied amount of air. The fall in the intermediate pressure does not allow proper suction pressure of the high-pressure stage compressor main body 2 to be achieved. Accordingly, the two-stage compressor lost an efficient pressure balance, leading to an inefficient operation. In addition, the pressure ratio in the high-pressure stage compressor main body 2 becomes large, and if accordingly, the power becomes excessive, the pressure required on the high-pressure stage side may be difficult to obtain or the operation may be impossible as rise in temperature becomes large.

In this embodiment, therefore, the ratio (n1/n2) of speed n1 of the low-pressure stage compressor main body 1 to speed n2 of the high-pressure stage compressor main body 2 is increased relative to the rating so as to raise the intermediate pressure. Specifically, based on the output of pressure sensors 21 and 22, the control unit 10 determines a rotational speed ratio so that the compression ratio in the low-pressure stage compressor main body 1 and the compression ratio in the high-pressure stage compressor main body 2 are made to be equal.

In Embodiment 1 described above, the compressor 20A can achieve three kinds of air supply modes, which are a supply of compressed air generated by driving the low-pressure stage compressor main body 1 alone, a supply of compressed air generated by driving the two-stage compressor main bodies 1 and 2, which is an ordinal supply, and a supply of compressed air generated by driving the two-stage compressor main bodies 1 and 2, and compressed air generated by driving the low-pressure stage compressor main body 1 alone.

In cases where the compressor 20A generates low-pressure compressed air, there is the advantage that energy loss is prevented by driving the low-pressure stage compressor main body 1 alone as compared with the case where high-pressure air is decompressed before use.

In addition, it is possible to simultaneously produce high-pressure compressed air as a two-stage compressor and low-pressure compressed air as a single stage machine.

Furthermore, one two-stage compressor has a configuration such that it can be used as a single stage compressor, a two-stage compressor, and a single-stage/two-stage simultaneously driven compressor, and achieves users' advantage relating to installation location and cost aspect, and also manufacturers' merits such as a reduction in the number of parts.

Embodiment 2

A compressor 20B according to Embodiment 2 of the present invention will be described. The main difference from Embodiment 1 is that the high-pressure stage suction valve 14 of Embodiment 1 is not provided and a by-pass pipe 25 is connected between a pipe between the low-pressure stage discharge check valve 19 in the low-pressure air line 100 side piping system and the low-pressure line discharge valve 13, and a pipe from the discharge side of the high-pressure stage compressor main body 1, a discharge side by-pass valve 26 is provided midway of this by-pass pipe 25, and in addition, in the discharge side pipe of the high-pressure stage compressor main body 2, a pressure regulating check valve 27 is provided downstream of the branch point with the by-pass pipe 25.

Further, the compressor 20B of Embodiment 2 is capable of supplying the compressed air generated by the low-pressure stage compressor main body 1 alone to the low-pressure air line and can perform the normal operation of the two-stage compressor. It does not operate to supply compressed air to both the high-pressure air line and the low-pressure air line by driving both compressor main bodies. The other configuration is similar to that of Embodiment 1. The same reference numerals are used for the same members and elements. Detailed description will be omitted.

Although the high-pressure stage suction valve 14 of Embodiment 1 invites a slight pressure loss of the compressed air which passes through it, in cases of the configuration of Embodiment 2, there is the advantage that such pressure loss does not occur at the time of the normal operation of the two-stage compressor.

The pressure regulating check valve 27 is a check valve which restricts flow of compressed air when the pressure is lower than a predetermined pressure. In this embodiment, in the pressure environment in which only the low-pressure stage compressor main body 1 is driven, the pressure regulating check valve 27 serves as “closed”, and it serves as “opened” in the high-pressure discharge environment during the normal operation of the two-stage compressor.

The discharge side by-pass valve 26 is an electromagnetic valve, and is controlled by the control unit 10. The discharge side by-pass valve 26 drives the low-pressure stage compressor main body 1 alone, and in cases where it supplies compressed air to the low-pressure air line, it serves as “opened”, and serves as “close” at the time of the normal operation of the two-stage compressor.

Embodiment 3

A compressor 20C according to Embodiment 3 of the present invention will be described. The main differences between Embodiment 3 and other embodiments is that when compressed air is supplied to the low-pressure air line alone, either one of the low-pressure stage compressor main body 1 or the high-pressure stage compressor main body 2 is selectively operated so that compressed air can be supplied.

When compressor air is supplied only to the low-pressure air line, in cases where the amount of air used is small, it is generally more efficient to operate the high-pressure stage compressor with a smaller volume at a higher speed than operating the low-pressure stage compressor at a low speed.

FIG. 4 shows a schematic configuration of the compressor 20C according to Embodiment 3. In the following description, the same reference numerals are used for the same members and elements, and a detailed description will be omitted.

The suction side piping system of the low-pressure stage compressor main body 1 branches. One pipe serves as a suction system of the low-pressure stage compressor main body 1, and another pipe 40 serves as a suction pipe connected so that it may communicate with the intermediate pipe. The pipe 40 may be connected to the suction side of the high-pressure stage compressor main body 2 without being connected to the intermediate pipe. The pipe 40 is provided with a solenoid valve (valve body) 36, and the opening and closing of the pipe 40 is controlled by the control unit 10 so that the flow of gas in the pipe 40 is permitted or prohibited.

As with Embodiment 2, Embodiment 3 has a by-pass pipe 25 branched from the discharge side piping system of the high-pressure stage compressor main body 2 to extend to the low-pressure air line 100. This embodiment has a configuration such that the by-pass pipe 25 branches from the outlet or the middle of aftercooler 7 to extend to the low-pressure air line 100.

In the compressor 20C having the above configuration, when supplying the compressed air only to the low-pressure air line, in cases where the amount of air is large, the low-pressure stage compressor main body 1 alone is operated whit the low-pressure line discharge valve 13 “opened”, and the electromagnetic valve 36 and the by-pass valve 26 “opened”. On the other hand, when the amount of air is small, in the compressor 20, the high-pressure stage compressor main body 2 alone is operated with the low-pressure line discharge valve 13, the electromagnetic valve 36 and the by-pass valve 26 “closed”.

Namely, the high-pressure stage compressor main body 2 sucks the outside air which does not pass through the discharge port of the low-pressure stage compressor main body 1, and generates the compressor air supplied to the low-pressure air line 100.

When high-pressure compressed air is supplied to the high-pressure air line alone by the normal operation of the two-stage compressor, the compressor 20C makes all the low-pressure line discharge valve 13, the electromagnetic valve 36, and the by-pass valve 26 “closed”. Further, when air is supplied to both air lines, the low-pressure line discharge valve 13 is “opened”, the electromagnetic valve 36 and the by-pass valve 26 are “closed”, and operation is performed by changing the operation speed ratio of the low-pressure stage compressor main body 1 and the high-pressure stage compressor main body 2 according to the amount of used air.

Thus, according to the compressor 20C of Embodiment 3, when compressed air is supplied to the low-pressure air line alone, either the low-pressure stage compressor main body 1 or the high-pressure stage compressor main body 2 is selected according to the amount of compressed air to supply, and the compressed air can be supplied more efficiently.

Although examples for carrying out the present invention have been described above, the present invention is not limited to the above-described various configurations and the like, and various modifications are possible without departing from the spirit thereof. It is also possible to replace the configuration of a specific embodiment with other embodiments. For example, in the above embodiment, the low-pressure line discharge valve 13 and the like are controlled by the control unit 10, but instead, a part or the whole of the electromagnetic valve may be a manual valve body, the operation may be such that compressed air is supplied to the low-pressure air line by the switching operation by the user. 

1. A multistage compressor comprising: at least a first compressor main body and a second compressor main body configured to suck compressed gas discharged from the first compressor main body to discharge the compressed gas with higher pressure; a first driving source configured to drive the first compressor main body; a second driving source configured to drive the second compressor main body; an intermediate pipe configured to connect a discharge side of the first compressor main body and a suction side of the second compressor main body; a low-pressure side discharge piping system branched from the intermediate pipe; an on-off valve disposed in the low-pressure side discharge piping system and configured to switch permission and prohibition of a flow of compressed gas discharged from the first compressor main body; and a control unit configured to control driving of the first driving source and the second driving source, wherein the control unit is configured to drive the first driving source alone when the on-off valve is opened.
 2. The multistage compressor according to claim 1, wherein the intermediate pipe includes a suction side on-off valve configured to switch permission and prohibition of the flow of compressed gas discharged from the first compressor main body to the suction side of the second compressor main body and, the control unit is configured to drive the first driving source alone when the on-off valve is opened, and the suction side on-off valve is closed.
 3. The multistage compressor according to claim 1, wherein the control unit is configured to further drive the second driving source, when the on-off valve is opened.
 4. The multistage compressor according to claim 1, comprising: a by-pass pipe configured to connect a discharge pipe of the second compressor main body and a downstream side of the on-off valve disposed at the low-pressure side discharge pipe, and a by-pass valve configured to switch permission and prohibition of a flow of compressed gas into the by-pass pipe, the compressed gas flowing through the discharge pipe, wherein the control unit is configured to drive the first driving source alone when the by-pass valve is opened, and the suction side on-off valve is closed.
 5. The multistage compressor according to claim 1, wherein at least one of the on-off valve, the suction side on-off valve, and the by-pass valve is an electromagnetic valve, and the control unit is configured to control the electromagnetic valve.
 6. A multistage compressor comprising: at least a first compressor main body and a second compressor main body configured to suck compressed gas discharged from the first compressor main body to discharge the compressed gas with higher pressure; a first driving source configured to drive the first compressor main body; a second driving source configured to drive the second compressor main body; an intermediate pipe which connects a discharge side of the first compressor main body, and a suction side of the second compressor main body; a low-pressure side discharge piping system branched from the intermediate pipe; an on-off valve disposed in the low-pressure side discharge piping system and configured to switch permission and prohibition of a flow of compressed gas discharged from the first compressor main body; a by-pass pipe configured to connect a discharge pipe of the second compressor main body and a downstream side of the on-off valve of the low-pressure side discharge pipe; a by-pass valve configured to switch permission and prohibition of a flow of compressed gas into the by-pass pipe, the compressed gas flowing through the discharge pipe; a suction pipe connected to the intermediate pipe or a suction side of the second compression main body and in which gas flows without passing through the first compressor main body; a valve body disposed at the suction pipe, and configured to permit or prohibit a flow of the gas; and a control unit configured to control driving of the first driving source and the second driving source, wherein the control unit configured to drive the second driving source alone when the on-off valve, the by-pass valve, and the valve body are opened.
 7. The multistage compressor according to claim 6, wherein the control unit is configured to drive the first driving source alone in place of the second driving source when the valve body is closed and the on-off valve and the by-pass valve are opened.
 8. The multistage compressor according to claim 6, where in the control unit is configured to drive the first driving source also when the by-pass valve and the valve body is closed, and the on-off valve is opened.
 9. The multistage compressor according to claim 6, wherein at least one of the on-off valve, the by-pass valve, and the valve body is an electromagnetic valve, and the control unit is configured to control the electromagnetic valve.
 10. The multistage compressor according to claim 2, wherein at least one of the on-off valve, the suction side on-off valve, and the by-pass valve is an electromagnetic valve, and the control unit is configured to control the electromagnetic valve.
 11. The multistage compressor according to claim 3, wherein at least one of the on-off valve, the suction side on-off valve, and the by-pass valve is an electromagnetic valve, and the control unit is configured to control the electromagnetic valve.
 12. The multistage compressor according to claim 4, wherein at least one of the on-off valve, the suction side on-off valve, and the by-pass valve is an electromagnetic valve, and the control unit is configured to control the electromagnetic valve.
 13. The multistage compressor according to claim 7, wherein at least one of the on-off valve, the by-pass valve, and the valve body is an electromagnetic valve, and the control unit is configured to control the electromagnetic valve.
 14. The multistage compressor according to claim 8, wherein at least one of the on-off valve, the by-pass valve, and the valve body is an electromagnetic valve, and the control unit is configured to control the electromagnetic valve. 